U.S. patent application number 10/900988 was filed with the patent office on 2005-03-17 for skew adjusted data cable.
Invention is credited to Clark, William T..
Application Number | 20050056454 10/900988 |
Document ID | / |
Family ID | 34118834 |
Filed Date | 2005-03-17 |
United States Patent
Application |
20050056454 |
Kind Code |
A1 |
Clark, William T. |
March 17, 2005 |
Skew adjusted data cable
Abstract
A twisted pair cable wherein characteristics of the twisted
pairs, such as twist lay, insulation thickness, characteristic
impedance, etc. are selected so as to achieve minimal skew between
the twisted pairs. In some examples, insulation materials may be
varied among the twisted pairs and composite insulations may be
used for one or more pairs in a cable.
Inventors: |
Clark, William T.;
(Lancaster, MA) |
Correspondence
Address: |
LOWRIE, LANDO & ANASTASI
RIVERFRONT OFFICE
ONE MAIN STREET, ELEVENTH FLOOR
CAMBRIDGE
MA
02142
US
|
Family ID: |
34118834 |
Appl. No.: |
10/900988 |
Filed: |
July 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60490651 |
Jul 28, 2003 |
|
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60553758 |
Mar 17, 2004 |
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Current U.S.
Class: |
174/113R |
Current CPC
Class: |
H01B 11/02 20130101;
H01B 7/0216 20130101 |
Class at
Publication: |
174/113.00R |
International
Class: |
H01B 011/02 |
Claims
1. A cable comprising: a first twisted pair of conductors
surrounded by a first insulation material having a first dielectric
constant, the first twisted pair of conductors having a first
signal phase delay; and a second twisted pair of conductors
insulated by a second insulation material having a second
dielectric constant greater than the first dielectric constant, the
second twisted pair of conductors having a second signal phase
delay substantially equal to the first signal phase delay such that
a skew of the cable is less than approximately 7 nanoseconds per
100 meters; wherein the first twisted pair of conductors has a
first twist lay and the second twisted pair of conductors has a
second twist lay greater than the first twist lay; and wherein the
second insulation material comprises a first layer having a third
dielectric constant and a second layer having a fourth dielectric
constant such that the second dielectric constant is an effective
dielectric constant of a combination the first and second
layers.
2. A cable comprising: a first twisted pair of conductors insulated
by a first insulation material having a first dielectric constant,
the first twisted pair of conductors having a first signal phase
delay; and a second twisted pair of conductors insulated by a
second insulation material having a second dielectric constant
greater than the first dielectric constant, the second twisted pair
of conductors having a second signal phase delay substantially
equal to the first signal phase delay such that a skew of the cable
is less than approximately 7 nanoseconds per 100 meters; wherein
the first twisted pair of conductors has a first twist lay and the
second twisted pair of conductors has a second twist lay greater
than the first twist lay; and wherein the first insulation is a
composite formed of at least two different materials.
3. A cable having a specified characteristic impedance comprising:
a plurality of twisted pairs of insulated conductors designated
into a first group of twisted pairs and a second group of twisted
pairs; wherein each twisted pair designated into the first group of
twisted pairs has a first twist lay, a first insulation thickness
and a first nominal impedance; wherein each twisted pair designated
into the second group of twisted pairs has a second twist lay, a
second insulation thickness and a second nominal impedance; and
wherein a first combination of the first twist lay and the first
insulation thickness, and a second combination of the second twist
lay and the second insulation thickness are selected such that a
difference between the first nominal impedance and the second
nominal impedance is greater than about 2 Ohms and less than about
15 Ohms, and the cable has a skew of less than approximately 25 ns
per 100 m.
4. The cable as claimed in claim 3, wherein each of the plurality
of twisted pairs has a same insulation material.
5. The cable as claimed in claim 3, wherein the first and second
combinations are selected such that an impedance delta between the
first nominal impedance and the second nominal impedance is in a
range of about 8 Ohms to 15 Ohms.
6. A method of manufacturing a cable comprising a plurality of
twisted pairs of insulated conductors that are designated into two
groups wherein each twisted pair designated into the first group of
twisted pairs has a first twist lay, a first insulation material
and a first insulation thickness and wherein each twisted pair
designated into the second group of twisted pairs has a second
twist lay, a second insulation material and a second insulation
thickness, the method comprising: selecting a combination of the
first twist lay, the first insulation material and the first
insulation thickness such that the twisted pairs designated into
the first group have a first nominal impedance; and selecting a
combination of the second twist lay, the second insulation material
and the second insulation thickness such that the twisted pairs
designated into the second group have a second nominal impedance
that is at least 2 Ohms greater than the first nominal impedance
and such that a skew between the twisted pairs of the first group
and the twisted pairs of the second group is less than about 25 ns
per 100 m.
7. The method as claimed in claim 6, wherein the act of selecting
the combination of the second twist lay, the second insulation
material and the second insulation thickness includes selecting the
combination such that a delta between the second nominal impedance
and the first nominal impedance is in a range of about 8 Ohms to 15
Ohms.
Description
RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.
119(e) to U.S. Provisional Application Ser. No. 60/490,651,
entitled "LOW-SKEW, HIGH SPEED DATA CABLE," filed on Jul. 28, 2003,
and U.S. Provisional Application Ser. No. 60/553,758, entitled
"SKEW ADJUSTED DATA CABLE," filed on Mar. 17, 2004, both of which
are herein incorporated by reference in their entireties.
BACKGROUND OF INVENTION
[0002] 1. Field of Invention
[0003] The present invention is directed to twisted pair cables,
particularly those having twist lays, insulation thicknesses,
insulation materials, and performance variables, such as
characteristic impedance, that are optimized to achieve low
skew.
[0004] 2. Discussion of Related Art
[0005] High performance electrical cables are often used to
transmit electrical signals between devices or components of a
network. These cables typically include several pairs of insulated
conductors twisted together, generally in a double-helix pattern
about a longitudinal axis. Such an arrangement of insulated
conductors, referred to herein as "twisted pairs," facilitates
forming a balanced transmission line for data communications. One
or more twisted pairs may subsequently be bundled and/or bound
together to form a data communication cable.
[0006] Modern communication cables must meet electrical performance
characteristics required for transmission at high frequencies. The
Telecommunications Industry Association and the Electronics
Industry Association (TIA/EIA) have developed standards which
specify specific categories of performance for cable impedance,
attenuation, skew and crosstalk isolation. For example, one
standard for crosstalk or, in particular, crosstalk isolation, is
TIA/EIA-568-A, wherein a category 5 cable is required to have 38 dB
of isolation between the twisted pairs at 100 MHz and a category 6
cable is required to have 42 dB of isolation between the twisted
pairs at 100 MHz. Various cable design techniques have been used to
date in order to try to reduce crosstalk and to attempt to meet the
industry standards. In addition, if cables are to be used in
plenum, they must pass the Underwriter's Laboratory Standard 910
test, commonly referred to as the Steiner Tunnel test.
[0007] These specifications and requirements limit the selection of
insulation materials that may be used in communication cables.
Preferred insulation materials have been fluoropolymers because
these materials provide certain desirable electronic
characteristics, such as low signal attenuation and reduced signal
phase delay. In addition, communication cables having insulation
materials formed from fluoropolymers can pass the Steiner Tunnel
test. Examples of fluoropolymer insulation materials used in
communication cables include fluoroethylenepropylene (FEP),
ethylenechlorotrifluoroethylene (ECTFE), polyvinylidene fluoride
(PVDF) and polytetrafluoroethylene (PTFE).
[0008] However, fluoropolymer insulation materials also have
disadvantages such as is relatively high cost and limited
availability caused by the high demand for these materials.
Therefore, several communication cables have been developed that
replace some of the fluoropolymer insulation materials with certain
non-fluoropolymer insulation materials. For example, U.S. Pat. No.
5,841,072 to Gagnon, herein incorporated by reference, discloses a
twisted pair cable wherein each conductor of the twisted pair has a
dual-layer insulation, the first (inner) layer being a foamed
polyolefin including a flame retardant and the second (outer) layer
being a fluoropolymer. In another example, a cable construction may
comprise a mix of conductors, for example, with some conductors of
the cable insulated with a single layer of fluoropolymer materials
and others conductors in the same cable insulated with a single
layer of polyolefin materials.
[0009] It is known that as the dielectric constant of an insulation
material covering the conductors of a twisted pair decreases, the
velocity of propagation of a signal traveling through the twisted
pair of conductors increases and the phase delay added to the
signal as it travels through the twisted pair decreases. In other
words, the velocity of propagation of the signal through the
twisted pair of conductors is inversely proportional to the
dielectric constant of the insulation material and the added phase
delay is proportional to the dielectric constant of the insulation
material. Thus, using different insulation materials among
conductor pairs within a cable may cause a variation in the phase
delay added to the signals propagating through different ones of
the conductors pairs. It is to be appreciated that for this
specification the term "skew" is a difference in a phase delay
added to the electrical signal for each of the plurality of twisted
pairs of the communication cable. A skew may result from the
insulation material covering one twisted pair of conductors being
different than the insulation material covering another twisted
pair of conductors of a communication cable.
[0010] In addition, in order to impedance match a cable to a load
(e.g., a network component), a cable may be rated with a particular
"characteristic impedance." For example, many radio frequency (RF)
components may have characteristic impedances of 50 or 100 Ohms and
therefore, many high frequency cables may similarly be manufactured
with a characteristic impedance of 50 or 100 Ohms so as to
facilitate connecting of different RF loads. The characteristic
impedance of the cable may generally be determined based on a
composite of the individual nominal impedances of each of the
twisted pairs making up the cable. The nominal impedance of a
twisted pair may be related to several parameters including the
diameter of the wires of the twisted pairs making up the cable, the
center-to-center distance between the conductors of the twisted
pairs, which may in turn depend on the thickness of the insulating
layers surrounding the wires, and the dielectric constant of the
material used to form the insulating layers.
[0011] In conventional manufacturing, it is generally considered
more beneficial to design and manufacture twisted pairs to achieve
as close to the specified characteristic impedance of the cable as
possible, generally within plus or minus 2 Ohms. The primary reason
for this is to take into account impedance variations that may
occur during manufacture of the twisted pairs and the cable. The
further away from the specified characteristic impedance a
particular twisted pair is, the more likely a momentary deviation
from the specified characteristic impedance the input impedance of
at any particular frequency due to impedance roughness will exceed
limits for both input impedance and return loss of the cable.
[0012] Many of the same parameters of a twisted pair affect both
the characteristic impedance and the skew of a twisted pair cable.
Therefore, there needs to be a balance or trade-off created between
these parameters for the cable to meet all specified performance
requirements, such as return loss, skew and crosstalk.
SUMMARY OF INVENTION
[0013] According to one embodiment, a cable comprises a first
twisted pair of conductors surrounded by a first insulation
material having a first dielectric constant, the first twisted pair
of conductors having a first signal phase delay, and a second
twisted pair of conductors insulated by a second insulation
material having a second dielectric constant greater than the first
dielectric constant, the second twisted pair of conductors having a
second signal phase delay substantially equal to the first signal
phase delay such that a skew of the cable is less than
approximately 7 nanoseconds per 100 meters. The first twisted pair
of conductors has a first twist lay and the second twisted pair of
conductors has a second twist lay greater than the first twist lay,
and the second insulation material comprises a first layer having a
third dielectric constant and a second layer having a fourth
dielectric constant such that the second dielectric constant is an
effective dielectric constant of a combination the first and second
layers.
[0014] According to another embodiment, a cable comprises a first
twisted pair of conductors insulated by a first insulation material
having a first dielectric constant, the first twisted pair of
conductors having a first signal phase delay, and a second twisted
pair of conductors insulated by a second insulation material having
a second dielectric constant greater than the first dielectric
constant, the second twisted pair of conductors having a second
signal phase delay substantially equal to the first signal phase
delay such that a skew of the cable is less than approximately 7
nanoseconds per 100 meters. The first twisted pair of conductors
has a first twist lay and the second twisted pair of conductors has
a second twist lay greater than the first twist lay, and the first
insulation is a composite formed of at least two different
materials.
[0015] Another embodiment of a cable having a specified
characteristic impedance comprises a plurality of twisted pairs of
insulated conductors designated into a first group of twisted pairs
and a second group of twisted pairs, wherein each twisted pair
designated into the first group of twisted pairs has a first twist
lay, a first insulation thickness and a first nominal impedance,
wherein each twisted pair designated into the second group of
twisted pairs has a second twist lay, a second insulation thickness
and a second nominal impedance, and wherein a first combination of
the first twist lay and the first insulation thickness, and a
second combination of the second twist lay and the second
insulation thickness are selected such that a difference between
the first nominal impedance and the second nominal impedance is
greater than about 2 Ohms and less than about 15 Ohms, and the
cable has a skew of less than approximately 25 ns per 100 m.
[0016] In one example of the cable, each of the plurality of
twisted pairs has a same insulation material. In another example,
the first and second combinations are selected such that an
impedance delta between the first nominal impedance and the second
nominal impedance is in a range of about 8 Ohms to 15 Ohms.
[0017] According to another embodiment, there is provided a method
of manufacturing a cable comprising a plurality of twisted pairs of
insulated conductors that are designated into two groups wherein
each twisted pair designated into the first group of twisted pairs
has a first twist lay, a first insulation material and a first
insulation thickness and wherein each twisted pair designated into
the second group of twisted pairs has a second twist lay, a second
insulation material and a second insulation thickness, the method
comprising steps of selecting a combination of the first twist lay,
the first insulation material and the first insulation thickness
such that the twisted pairs designated into the first group have a
first nominal impedance, and selecting a combination of the second
twist lay, the second insulation material and the second insulation
thickness such that the twisted pairs designated into the second
group have a second nominal impedance that is at least 2 Ohms
greater than the first nominal impedance and such that a skew
between the twisted pairs of the first group and the twisted pairs
of the second group is less than about 25 ns per 100 m.
[0018] In one example, the act of selecting the combination of the
second twist lay, the second insulation material and the second
insulation thickness includes selecting the combination such that a
delta between the second nominal impedance and the first nominal
impedance is in a range of about 8 Ohms to 15 Ohms.
BRIEF DESCRIPTION OF DRAWINGS
[0019] The accompanying drawings, are not intended to be drawn to
scale. In the drawings, each identical or nearly identical
component that is illustrated in various figures is represented by
a like numeral. For purposes of clarity, not every component may be
labeled in every drawing. In the drawings:
[0020] FIG. 1 is a perspective view of a cable including two
twisted pairs having different twist lay lengths;
[0021] FIG. 2 is a schematic cross-sectional diagram of a twisted
pair of insulated conductors;
[0022] FIGS. 3A-3D are graphs illustrating impedance versus
frequency for twisted pairs of one embodiment of a cable;
[0023] FIGS. 4A-4D are graphs illustrating return loss versus
frequency for the same twisted pairs as in FIGS. 3A-3D;
[0024] FIG. 5 is a cross-sectional diagram of one embodiment of a
twisted pair cable according to aspects of the invention;
[0025] FIGS. 6A-6D are graphs illustrating impedance versus
frequency for twisted pairs of one embodiment of a cable;
[0026] FIGS. 7A-7D are graphs illustrating return loss versus
frequency for the same twisted pairs as in FIGS. 6A-6D; and
[0027] FIG. 8 is a cross-sectional diagram of another embodiment of
a twisted pair cable according to aspects of the invention.
DETAILED DESCRIPTION
[0028] Various embodiments of the invention are described in detail
below with reference to the accompanying figures. However, it is to
be appreciated that the invention is not limited to any number of
twisted pairs or any profile for the cables illustrated in any of
these embodiments. The inventive principles can be applied to
cables including greater or fewer numbers of twisted pairs and
having different core profiles. In addition, the invention is not
limited in its application to the details of construction and the
arrangement of components set forth in the following description or
illustrated in the drawings. The invention is capable of other
embodiments and of being practiced or of being carried out in
various ways. Also, the phraseology and terminology used herein is
for the purpose of description and should not be regarded as
limiting. The use of "including," "comprising," or "having,"
"containing", "involving", and variations thereof herein, is meant
to encompass the items listed thereafter and equivalents thereof as
well as additional items.
[0029] According to one embodiment, a cable 100 may comprise a
plurality of twisted pairs of insulated conductors including a
first twisted pair 102 and a second twisted pair 104, surrounded by
an outer jacket 106, as illustrated in FIG. 1. The outer jacket 106
may be any suitable jacket material, including, for example, a
polyvinylchloride (PVC), a low-smoke, low-flame PVC, or any plenum
or non-plenum rated thermoplastic. Each twisted pair of the
plurality of twisted pairs has a specified distance between twists
along the longitudinal direction, that distance being referred to
as the pair twist lay. When adjacent twisted pairs have the same
twist lay and/or twist direction, they tend to lie within a cable
more closely spaced than when they have different twist lays and/or
twist direction. Such close spacing increases the amount of
undesirable crosstalk which occurs between the adjacent pairs.
Therefore, each twisted pair within the cable 100 may have a unique
pair lay in order to increase the spacing between pairs and thereby
to reduce the crosstalk between the twisted pairs of the cable.
Twist direction may also be varied.
[0030] Referring to FIG. 1, the first twisted pair of conductors
102 includes two electrical conductors 108 each surrounded by an
insulation layer 110 of a first insulation material. The second
twisted pair of conductors 104 also includes two electrical
conductors 108 each surrounded by an insulation layer 112. As shown
in FIG. 1, the twisted pairs 102, 104 may have different twist lay
lengths to reduce unwanted crosstalk between the pairs. However,
the shorter a given pair's twist lay length, the longer the
"untwisted length" of that pair and thus the greater the signal
phase delay added to an electrical signal that propagates through
the twisted pair. It is to be understood that the term "untwisted
length" herein denotes the electrical length of the twisted pair of
conductors when the twisted pair of conductors has no twist lay
(i.e., when the twisted pair of conductors is untwisted).
Therefore, using different twist lays among the twisted pairs
within a cable may cause a variation in the phase delay added to
the signals propagating through different ones of the conductors
pairs.
[0031] As discussed above, both the insulation material used for
the insulated conductors and the twist lay used for each twisted
pair may affect the propagation velocity of electrical signals
through the twisted pairs. In order to reduce crosstalk between
pairs, it may be desirable to vary the twist lays of the twisted
pairs 102, 104. However, this may result in the twisted pairs 102,
104 having different electrical lengths, causing a skew to exist
within the cable 100. The present invention is directed to several
configurations of cables using varying twist lays and insulation
materials optimized to achieve closely matched signal velocities
relative to the final twist lays of the cable to minimize skew
within the cable.
[0032] As discussed above, the propagation velocity of a signal
through a twisted pair of insulated conductors is affected by the
dielectric constant of the insulating material used for that
twisted pair. For example, using a so-called "faster" insulation,
such as fluoroethylenepropylene (FEP), the propagation velocity of
a signal through the twisted pair 102 may be approximately 0.69 c
(where c is the speed of light in a vacuum). For a "slower"
insulation, such as polyethylene, the propagation velocity of a
signal through the twisted pair 102 may be approximately 0.66
c.
[0033] According to one embodiment, the second twisted pair 104 may
have a longer twist lay length than does the first twisted pair
102, as shown in FIG. 1. A shorter twist lay for a first twisted
pair of insulated conductors relative to a second twisted pair
results in the first twisted pair having a longer electrical length
than the second twisted pair, assuming the first and second twisted
pairs have a similar insulation material on the insulated
conductors. Therefore, by using a higher dielectric constant
material (slower insulation) for the second twisted pair (which has
a shorter electrical length due to its longer twist lay) relative
to the first twisted pair, the phase delay added to the electrical
signals propagating through the first and second twisted pairs may
be equalized. In this manner, the skew between the first and second
twisted pairs may be minimized.
[0034] Thus, the second twisted pair 104 may have the second
insulation layers 112 comprising a second insulation material that
has a higher dielectric constant than the first insulation
material. For example, the first insulation layer 110 may comprise
FEP and the second insulation layer 112 may comprise polyethylene.
Compensating for the higher signal phase delay provided by the
twisted pair 104 (due to the higher dielectric constant of the
insulation layer 112) relative to the twisted pair 102, the
untwisted length of the twisted pair 102 can be increased compared
to the untwisted length of the twisted pair 104. Thus, by
controlling the twist lay lengths of twisted pairs 102 and 104
relative to one another and by selecting insulation materials
having different dielectric constants for the insulation layers
110, 112, the signal phase delay added to the signal by the twisted
pair 102 can be manipulated to be similar to the signal phase delay
added to the signal propagating through the twisted pair 104.
[0035] The effective dielectric constant of an insulation material
may also depend, at least in part, on the thickness of the
insulating layer. This is because the effective dielectric constant
may be a composite of the dielectric constant of the insulating
material itself in combination with the surrounding air. Therefore,
the propagation velocity of a signal through a twisted pair may
depend not only on the twist lay and insulation material used, but
also on the thickness of the insulation of that twisted pair.
[0036] Referring to FIG. 2, there is illustrated a cross-sectional
view of one example of a twisted pair of insulated conductors. The
twisted pair 114 comprises two electrical conductors 116 which may
be, for example, metal wires or strands, each surrounded by at
least one insulating layer 118. The nominal impedance of a twisted
pair 114 may be related to several parameters including the
diameter of the conductors 116 of the twisted pairs making up the
cable, the center-to-center distance 120 between the conductors of
the twisted pairs, which may in turn depend on the thickness of the
insulating layers 118 and the dielectric constant of the material
used to form the insulating layers 118.
[0037] The characteristic impedance of the cable may generally be
determined based on a composite of the individual nominal
impedances of each of the twisted pairs making up the cable. The
nominal characteristic impedance of each twisted pair may be
determined by measuring the input impedance of the twisted pair
over a range of frequencies, for example, the range of desired
operating frequencies for the cable. A curve fit of each of the
measured input impedances, for example, for 801 measured points,
across the operating frequency range of the cable may then be used
to determine a "fitted" nominal characteristic impedance of each
twisted pair making up the cable, and thus of the cable as a whole.
The TIA/EIA specification for characteristic impedance of a cable
is given in terms of this fitted characteristic impedance including
an allowable range of deviation. For example, the specification for
a category 5 or 6,100 Ohm cable is 100 Ohms, +/-15 Ohms for
frequencies between 100 and 350 MHz and 100 Ohms +/-12 Ohms for
frequencies below 100 MHz.
[0038] In conventional cables, it is common to design the twisted
pair to have a nominal input impedance as close as possible to the
specified overall nominal input impedance of the cable. By
contrast, Applicant has identified that a reduction in the skew of
a cable can be obtained by optimizing the insulation thicknesses to
specific pair lays and, in this optimization procedure, allowing an
increased deviation of the nominal impedances of the twisted pairs
relative to the specified characteristic impedance value for the
cable. An advantage of selecting this trade-off is that reduced
skew can be obtained while still achieving an acceptable impedance
variation and return loss for the cable.
[0039] As stated above, the specification for the characteristic
impedance of a category 5 or category 6, 100 Ohm cable allows a
maximum deviation from the specified 100 Ohm impedance value of
+/-15 Ohms for operating frequencies between 100 and 350 MHz and
+/-12 Ohms for operating frequencies below 100 MHz. However,
conventionally, cable manufacturers have attempted to ensure that
each twisted pair has a nominal impedance within +/-2 Ohms of the
specified characteristic impedance of the cable. Modern
manufacturing includes computerized real-time process controls,
latest-technology equipment and improved raw materials, allowing
for greater precision in the manufacturing process. This enhanced
precision manufacturing allows for use of more of the 15 Ohm (or 12
Ohm) tolerance range because greater precision reduces the
"roughness" of the impedance over the operating frequency range.
Allowing greater variation in the nominal impedance of the twisted
pairs may allow optimization, or variation, of parameters affecting
characteristic impedance, to improve other performance
characteristics of the cable, such as, for example, the skew of the
cable. One example of a machine that may be used, in combination
with a standard extrusion machine, to achieve improved
manufacturing precision is a Beta LaserMike Model 1000 parameter
measuring machine. This machine may be used to measure cable
parameters during manufacture of the cable and information provided
by the machine can be sued to extrude twisted pairs with tighter
tolerances.
[0040] Referring to Table 1 below, there is given exemplary twist
lay lengths for each twisted pair in one example of a four-pair
cable. A conventional cable (including four twisted pairs having
the twist lay lengths given in Table 1) designed to have
characteristic impedance of about 100 Ohms and using like
insulation materials and thicknesses on each conductor of the four
twisted pairs, may typically have a skew of about 25 nanoseconds
(ns) per 100 meters (m) for faster insulations (for example, FEP @
0.69 c), and about 30 ns/100 m for slower insulation (e.g.,
polyethylene @ 0.66 c). Conventionally, the insulation thicknesses
would be selected (as shown in Table 1) to achieve an impedance
variation of about +/-1 to 2 Ohms among the twisted pairs.
1TABLE 1 Conventional Characteristic Twist Lay Length Insulation
Thickness Impedance Twisted Pair (inches) (inches) (Ohms) 1 0.504
0.043 100 .+-. 2 2 0.744 0.039 100 .+-. 2 3 0.543 0.043 100 .+-. 2
4 0.898 0.039 100 .+-. 2
[0041] Applicant has recognized that by optimizing the insulation
thicknesses relative to the twist lays of each twisted pair in the
cable, the skew of a cable can be substantially reduced. Although
varying the insulation thicknesses may cause variation in the
characteristic impedance values of the twisted pairs, under
improved manufacturing processes, impedance roughness over
frequency (i.e., variation of the impedance of any one twisted pair
over the operating frequency range) can be controlled to be
reduced, thus allowing for a design optimized for skew while still
meeting the specification for characteristic impedance and return
loss.
[0042] According to one embodiment, a four-pair cable was designed,
using a slower insulation material (e.g., polyethylene) and
standard pair lays, where all insulation thicknesses were set to
0.041 inches. The twist lays are given below in Table 2. This cable
exhibited a skew reduction of about 8 ns/100 meters (relative to
the conventional cable described above--this cable was measured to
have a worst case skew of approximately 21 ns, whereas the
conventional, impedance-optimized cable exhibits a skew of
approximately 30 ns or higher), yet the individual pair impedances
were within 0 to 3 Ohms of deviation from the specified
characteristic impedance (as shown in Table 2), leaving plenty of
room for further impedance deviation, and therefore skew
reduction.
2TABLE 2 Twist Lay Length Thickness of Insulation Nominal Twisted
Pair (inches) (inches) Impedance 1 0.504 0.041 100 2 0.744 0.041
102 3 0.543 0.041 99 4 0.898 0.041 103
[0043] According to another embodiment of the invention, a cable
may comprise a plurality of twisted pairs of insulated conductors,
wherein twisted pairs with longer pair lays have a relatively
higher characteristic impedance and larger insulation thickness,
while twisted pairs with shorter pair lays have a relatively lower
characteristic impedance and smaller insulation thickness. In this
manner, pair lays and insulation thickness may be controlled so as
to further reduce the overall skew of the cable. One example of
such a cable, using polyethylene insulation is given in Table 3
below. This cable was measured to have a skew of approximately 17
ns.
3TABLE 3 Twist Lay Length Thickness of Insulation Nominal Twisted
Pair (inches) (inches) Impedance 1 0.504 0.042 97 2 0.744 0.040 103
3 0.543 0.041 97.5 4 0.898 0.040 103
[0044] This concept may be better understood with reference to
FIGS. 3A-D and 4A-D which respectively illustrate graphs of
measured input impedance versus frequency and return loss versus
frequency for the twisted pairs of the four-pair cable described in
Table 2. Referring to FIGS. 3 A-D, the "fitted" characteristic
impedance, line 200, for each twisted pair (over the operating
frequency range) may be determined from the measured input
impedance, line 202, over the operating frequency range. Lines 204
indicate the category 5/6 specification range for the input
impedance of the twisted pairs. As shown in FIGS. 3A-D, the
measured input impedance 202 of each of the twisted pairs 1-4 falls
within the specified range (within lines 204) over the entire
specified operating frequency range of the cable. As shown in FIGS.
3A-3D, the category 5 or category 6, 100 Ohm cable specification
allows a maximum deviation from the specified 100 Ohm impedance
value of +/-15 Ohms for operating frequencies between 100 and 350
MHz and +/-12 Ohms for operating frequencies below 100 MHz, shown
by the discontinuities 208 in lines 204.
[0045] Referring to FIGS. 4 A-D, there are illustrated
corresponding return loss versus frequency plots for each of the
twisted pairs. The lines 210 indicates the category 5/6
specification for return loss of the twisted pairs over the
operating frequency range. As shown in FIGS. 4A-4D, the measured
return loss 120 for each of twisted pairs 1-4 is above the
specified limit (and thus within specification) over the entire
specified operating frequency range of the cable. Thus, the
characteristic impedance of at least some of the twisted pairs
could be allowed to deviate further from the desired 100 Ohms, if
necessary, to reduce further skew. In other words, the twist lays
and insulation thicknesses of the twisted pairs may be further
varied to reduce the skew of the cable while still meeting the
impedance specification.
[0046] One aspect of this disclosure is allowing some deviation in
the twisted pair characteristic impedances relative to the nominal
impedance value to allow for a greater range of insulation
thicknesses. Smaller diameters are provided for a given pair lay to
result in a lower pair angle and shorter non-twisted pair length.
Conversely, larger pair diameters result in a higher pair angles
and longer non-twisted pair length. Where a tighter (shorter) pair
lay would normally have an insulation thickness of 0.043 inches for
100 Ohms, a diameter of 0.041 inches yields a reduced impedance of
about 98 Ohms. Longer pair lays using the same insulation material
would normally have a lower insulation thickness of about 0.039
inches for 100 ohms, and a diameter of 0.041 inches can be provided
and yield about 103 Ohms. As shown in FIGS. 3A-D and 4A-D, allowing
this "target" impedance variation from 100 Ohms does not prevent
the twisted pairs, and the cable, from meeting the input impedance
specification, but may allow improved skew in the cable.
[0047] As discussed above, the many constraints imposed on cable
designs by the industry standards and specifications may limit the
variety of materials that may be used as insulation for the
conductors of the twisted pairs. This may, in turn, limit the
accuracy with which the signal phase delay added by each twisted
pair may be controlled, or may impose strict tolerances on the
twist lays of each twisted pair. Applicants have recognized that by
using dual-layer insulation for at least some of the twisted pairs
may allow the added signal phase delay to be controlled with better
precision, at least in part because the effective dielectric
constant of the dual-layer insulation depends upon the dielectric
constant of the materials used for each layer and on the ratio of
the relative thickness of each layer.
[0048] According to one embodiment, illustrated in FIG. 5, the
cable 40 may include four twisted pairs of insulated conductors
250a-d, each twisted pair including two electrical conductors 252
surrounded by an insulation. The twisted pairs may be surrounded by
a jacket 258 to form the cable 40. In one example, two twisted
pairs 250a, 250d may have a dual-layer insulation and two twisted
pairs 250b, 250c may have single-layer insulation. It is to be
appreciated that the principles of the invention are not limited to
a four pair cable and may be applied to twisted pair cables
comprising more or fewer than four twisted pairs of conductors. In
addition, although the illustrated example includes two twisted
pairs having dual-layer insulation, the invention is not so
limited, and one, a plurality or all of the twisted pairs may have
dual-layer insulation.
[0049] According to one embodiment, the dual-layer insulation of at
least one twisted pair, for example, twisted pair 250a, may
comprise a first insulation layer 254 and a second insulation layer
256. In one example, the first insulation layer 254 may be a
polyolefin-based material, such as, for example, polyethylene's,
polypropylenes, flame retardant polyethylene, and the like. The
second insulation layer 256 may be, for example, FEP or another
fluoropolymer. As discussed above, using a fluoropolymer for the
outer (second) insulation layer may have advantages in terms of
passing the Steiner Tunnel test so that the cable may be plenum
rated. However, the invention is not limited to plenum rated
cables, and the second insulation layer 256 may also be a
non-fluoropolymer. The thicknesses of the first and second
insulation layers may be chosen according to factors such as
relative cost of the materials and the smoke and flame properties
of the materials. The ratio between the thickness of the first
insulation layer 254 and the second insulation layer 256 may be
selected based on the dielectric constants of the material used for
each layer and the desired overall effective dielectric constant
for the dual-layer insulation.
[0050] Referring again to FIG. 5, at least one twisted pair, for
example, twisted pair 250b, may comprise a single insulation layer
260 that may be, for example, solid FEP. Table 3 below provides
dimensions for one specific example of a four pair cable according
to the invention wherein two twisted pairs have a single insulation
layer of FEP and the other two twisted pairs have dual-layer
insulation, the inner layer being a flame retardant polyethylene
and the outer layer being FEP. The worst-case skew (i.e., the
largest skew between any two twisted pairs) for this exemplary
cable was measured to be approximately 4.45 ns/100 meters.
4TABLE 4 1st 2nd Twist Lay Solid Insulation Insulation Insulation
Length (FEP) Layer Layer Twisted Pair (inches) (inches) (inches)
(inches) Blue (50b) 0.507 0.0385 -- -- Orange (50a) 0.698 -- 0.0275
0.0368 Green (50c) 0.543 0.0380 -- -- Brown (50d) 0.776 -- 0.0275
0.0368
[0051] It is to be appreciated that the above dimensions and
specified materials are provided as an example for the purposes of
explanation and that the invention is not limited to the specifics
examples given herein. In particular, considering the four-pair
cable illustrated in FIG. 5, the twisted pair 250c may have a
single-layer insulation 266 that is not the same material as
insulation layer 260 of twisted pair 50b. Furthermore, twisted pair
50d may have a dual-layer insulation that comprises a first layer
268 and a second layer 270, the thicknesses of which may be
different from the thicknesses of the insulation layers 256 and 256
used on twisted pair 50a.
[0052] Referring to FIGS. 6A-D, there is illustrated measured
impedance versus frequency of each of the twisted pairs given in
Table 4. The measured impedance is indicated by lines 220. The
boundary lines 222 indicate the maximum tolerances (i.e.,
deviations from the specified 100 Ohms target impedance) allowed by
the category 5/6 specifications. Again, the discontinuities 224 in
the lines 222 illustrate that the allowed tolerances vary with
frequency. As can be seen from FIGS. 6A-D, the measured impedance
of each of the twisted pairs falls within the specified tolerances
over the specified operating frequency range of the cable. FIGS.
7A-d illustrate graphs for each of the twisted pairs of Table 4
showing return loss versus frequency. The return loss for each
twisted pair is indicated by lines 226. The category 5/6 return
loss specification is indicated by lines 228. As can be seen in
FIGS. 7A-D, the measured return loss of each twisted pair meets the
category 5/6 specification.
[0053] The skew between each twisted pair combination for the
above-described cable was measured and is given in Table 5 below.
As discussed above, the worst-case skew (i.e., the largest skew
between any two twisted pairs) for this exemplary cable was
measured to be approximately 4.48 ns/100 meters, illustrating that
such a cable can achieve a significant improvement in skew over a
conventional cable.
5 TABLE 5 Measured Skew Twisted Pair Combination (ns/100 m)
Blue-Orange 1.67 Blue-Green 2.65 Blue-Brown 4.48 Orange-Green 1.44
Orange-Brown 2.83 Green-Brown 1.97
[0054] Referring to FIG. 8, there is illustrated another embodiment
of the invention wherein a cable 70 may comprise a plurality of
twisted pairs of insulated conductors surrounded by an outer jacket
72. Each twisted pair comprises two conductive cores 74 each
surrounded by an insulation layer. At least one twisted pair 76a
may have insulation layers 78a formed from a material that has a
dielectric constant different from that of the material used to
form insulation layers 78b of another twisted pair 76b. The ratio
of the dielectric constants of the materials of insulation layers
78a and 78b may be varied to achieve closely matched signal phases
between twisted pairs 76a and 76b relative to the final twist lays
of twisted pairs 76a and 76b. Preferably, the worst case skew
between any twisted pair 76a and twisted pair 76b may be less than
approximately 7 ns/100 meters, and most preferably less than 5
ns/100 meters.
[0055] According to another embodiment, the insulation layer for at
least one of the plurality of twisted pairs in the cable may
comprise an extruded composite insulation layer 78c. A plurality of
materials may be combined and mixed during the extrusion process to
form the single layer composite insulation 78c. At least one of the
materials used to form the composite insulation 78c may have a
dielectric constant that is different from the dielectric constant
the insulation material on one or more conductors of at least one
other twisted pair in the cable.
[0056] In one example, the materials that may be mixed to provide
the composite insulation may be polyolefins. The ratio of volumes
of the various materials used to form the composite insulation may
be selected so as to provide a composite insulation having a
desired effective dielectric constant and desired effective
propagation velocity characteristics. For example, a first material
may have a velocity characteristic v1=0.66 c (where c is the speed
of light in a vacuum) and a second material may have a velocity
characteristic v2=0.68 c. If the first and second materials are
mixed in equal quantities, they may yield a composite material
having a velocity characteristic vm=0.67 c. Therefore, by
controlling the materials used and the ratio of volumes in which
they are mixed, a composite material may be formed having a
predetermined desired velocity characteristic and effective
dielectric constant.
[0057] One or more twisted pairs of insulated conductors in a
multi-pair cable may use a composite insulation material, as
described above, such that a ratio of the effective dielectric
constants of the materials relative to another twisted pair within
the cable may be varied to achieve closely matched signal
velocities relative to the final twist lays of the twisted
pairs.
[0058] According to one embodiment, a four-pair cable, such as
illustrated in FIG. 6, may comprise two twisted pairs having
relatively shorter (although not necessarily identical) twist lays
and two twisted pairs having relatively longer (although not
necessarily identical) twist lays. The insulation used for the two
twisted pairs having the shorter twist lays may have a faster
velocity characteristic than the insulation used for the two
twisted pairs having the longer twist lays. Each insulation may be
formed from a composite mixture of materials, mixed in
predetermined ratios to obtain the desired velocity
characteristics. In other words, the composite insulation materials
used on the different twisted pairs may be optimized for the
different twist lays such that the skew between any two twisted
pairs may be less than approximately 7 ns/100 m and preferably less
than 5 ns/100 m.
[0059] Table 4 below provides a theoretical example of one
embodiment of a four pair cable using composite insulations. The
composite insulation is formed from a mixture, in the proportions
given in the table below, of a first insulation material having a
velocity characteristic of 0.66 c and a second insulation material
having a velocity characteristic of 0.61 c. A cable according to
this example theoretically has a skew of less than approximately 5
ns/100 m.
6TABLE 6 Twist Composite Lay Insulation 1st Insulation 2nd
Insulation Length Diameter .66 c (% of .61 c (% of Twisted Pair
(inches) (inches) composite) composite) Blue (50b) 0.507 0.040 100
0 Orange (50a) 0.698 0.0385 45 065 Green (50c) 0.543 0.0395 83 17
Brown (50d) 0.776 0.0385 0 100
[0060] In one example, a multiple pair cable may comprise a
plurality of twisted pairs of insulated conductors, at least one
twisted pair having an insulation material that is different from
the insulation material of another twisted pair, wherein the
insulation thicknesses may be optimized for a skew less than
approximately 7 ns/100 meters. In another example, the insulation
thicknesses may be optimized for a skew less than approximately 25
ns/100 meters. In yet another example, the insulation thicknesses
may be optimized for a characteristic impedance deviation among the
twisted pairs of less than about 15 Ohms. By selecting slower of
faster dielectrics for the insulation and optimizing the thickness
of the selected insulation, the impedance variation between twisted
pairs can be reduced for any given desired skew value. For example,
a faster insulation material, such as FEP, may allow a twisted pair
with a shorter twist lay length to have slightly thicker insulation
layer, e.g., about 2 mils thicker, relative to another twisted pair
with a longer twist lay length, the two twisted pairs still
maintaining desired skew results. In summary, all parameters,
including insulation material, twist lay length and insulation
thickness, may be individually adjusted to obtain desired skew and
return loss performance.
[0061] Having thus described several aspects of at least one
embodiment of this invention, it is to be appreciated various
alterations, modifications, and improvements will readily occur to
those skilled in the art. Such alterations, modifications, and
improvements are intended to be part of this disclosure, and are
intended to be within the spirit and scope of the invention.
Accordingly, the foregoing description and drawings are by way of
example only.
[0062] What is claimed is:
* * * * *